|
Fulgurites (from the Latin ''fulgur'', meaning "lightning") are classified generically as a variety of the mineraloid lechatelierite, although their absolute chemical composition is dependent on the physical and chemical properties of target material affected by the discharge of cloud-ground lightning. They are commonly hollow and/or branching assemblages of glassy, protocrystalline, and heterogeneously-microcrystalline tubes, crusts, slags, vesicular masses, and clusters of refractory materials that often form during the discharge phase of lightning strikes propagating into silica-rich quartzose sand, mixed soil, clay, caliche and other carbonate-rich sediments, humic sediments, conductive biomass (such as peat, water-saturated wood, or dung), or anthropogenic materials having similar compositions (e.g. concrete, brick, asphalt, tile, etc.). Colloquially, they have been referred to as petrified lightning. Fulgurites are homologous to Lichtenberg figures, which are the branching patterns produced on surfaces of insulators during dielectric breakdown by high-voltage discharges, such as lightning. ==Description== Fulgurites are formed when lightning with a temperature of at least melts silica or other common conductive and semiconductive minerals and substrates, fusing, vitrifying, oxidizing and reducing mineral grains and organic compounds; the fulgurite mass is the rapidly-quenched end-product. The temperature peak within a lightning channel, however, is known to exceed 30,000 K, with sufficient pressure to produce planar deformation features, or "shock lamellae" in SiO2 polymorphs.〔()〕〔() 〕 It is assumed that the process of forming a fulgurite occurs over a timespan of the order of a single second,〔 following the termination of the return stroke sequence, and leaves direct evidence of the dissipation path and its dispersion over the surface or into the earth. Artificial fulgurites can also be produced when the controlled arcing of electricity into a fusable medium. This has been documented in cases of downed high voltage power lines; current was discharged into the ground, producing blue fulgurite-like lechatelierites, colored by copper from the power line.〔url=http://agatelady.blogspot.com/2014/09/more-about-fulgurites.html〕 Fulgurites occasionally form as glazed tracks on rock, or as networks completely metamorphosing the target rock. Ejected droplets and irregular sub-rounded structures are associated with such rock (type IV) fulgurites, as well as type II (mixed soil) and clay fulgurites; these unmixed and processed ejected materials, often quench into bizarre forms - typically aerodynamically-contoured and displaying some degree of structural complexity, are classified as ''exogenic fulgurites'' (Type V), and can resemble many confirmed crater glass forms, impactites, and tektites.〔(Exogenic fulgurites from Elko County, Nevada: a new class of fulgurite associated with large soil-gravel fulgurite tubes ) (''Rocks & Minerals'', Sep/Oct 2004, Vol. 79, No. 5.)〕 The color of fulgurites varies widely, depending on composition and chemical impurities. It can range from black or tan, to green, blue, metallic blue-grey, or a translucent white. More colorful variants are usually synthetic and reflect incorporation of synthetic materials. The interior of Type I (sand) fulgurites normally is very smooth or lined with fine bubbles, while other types are often both vesicular and dense or porous and scoria-like; their exteriors generally can be coated with rough sedimentary particles and can be porous, smooth, or structurally-complex. Fulgurites display some degree of self-similarity and structural scale invariance as a macroscopic or microscopic network of root-like branches. Fulgurites formed in sand or loose soil are mechanically fragile, making the field collection of large specimens difficult. The primary SiO2 phase in fulgurites is lechatelierite, a silica glass. Because their groundmass is generally amorphous in structure, fulgurites are classified as mineraloids. Fulgurites can exceed tens of centimeters in diameter and can penetrate deep into the subsoil, sometimes occurring as far as below the surface that was struck, but may form directly on appropriate sedimentary surfaces. One of the longest fulgurites to have been found in modern times was a little over in length, and was found in northern Florida. The Yale University Peabody Museum of Natural History displays one of the longest known preserved fulgurites, approximately in length. Charles Darwin in ''The Voyage of the Beagle'' recorded that tubes such as these found in Drigg, Cumberland, UK reached a length of . The Winans Lake fulgurite() (Winans Lake, Livingston County, Michigan), extended discontinuously throughout a 30 m range, and arguably includes the largest reported fulgurite mass ever recovered and described - its largest section extending approximately 16 ft (4.88 m) in length by 1 ft in diameter (30 cm).〔 抄文引用元・出典: フリー百科事典『 ウィキペディア（Wikipedia）』 ■ウィキペディアで「fulgurite」の詳細全文を読む スポンサード リンク
|